Chainsaw Repair

I have a cylinder that has too much of squish area, that is, vertically.
The resulting lower compression is preventing a clean burn of the 2 stroke oil.
I am think of removing some metal from the cylinder base and thus get the piston to go higher for more compression.
The side effects are advancing of the exhaust port and retarding of the inlet port.
Is this going to have a negative effect?
I think I need to grind off about  1mm from the cylinder base.

Joe

You won't change the port timing that much by lowering the cylinder .The degrees will remain the same as far as "blow down " between the opening of the exhaust and opening of the transfers .The intake will advance slightly and the exhaust will be retarded slightly .Probabley around three degrees which if anything will increase the torque but might slightly decrease the top end speed .

With the increased torque most likely it won't even be noticed .

FWIW most older Stihls set the exhaust port opening at around 98-100 degrees .If you wanted to you can raise it up slightly to maybe 96 degrees which will give you more blow down and slightly more RPM's .If you only raise the compression slightly I personally would not change the actual port timing numbers but might widen the ports slightly .

More info the older models used maybe 45-46 thou clearance or squish and the gasket is a flat 20 thou thick .If you use sealant that might account for 2 thou thereby reducing the clearance by around 18 thou more or less .

There's a certain formula that takes into account the port time open ratios depending on the bore size and RPM's .MacDizzy ,Bell ,and Gorden Jennings might be some help as I can't remember them exactly myself .

Early on I basically did about the same and it did cut faster .Although instead of sealant I cut the cylinder bases on a lathe .It was only later with a couple of models I later went in and did some tweeking to the ports to jack up the  speed a tad bit .

You know this method really isn't a bad way to jack some power with very little effort .This and a little muffler work would provide very satisfactory to 90 percent of folks who want just a little more grunt but don't feel like ponying up a couple hundred to get someone to soup it up .Once again proving you don't have to be a bloody genious to pep up a saw engine .

What the heck if it didn't work out just put the gasket back in and you're right back to where you started .Full circle so to speak .

Okay Al, EPH,
I did get my ports crossed.
The exhaust port will be retarded because the piston top opens it a bit later now.
The Inlet port gets advanced because it is now a bit lowered and the lower end of the piston opens it a bit earlier now.

Now, coming to the transfer ports, they would have a retarded timing because the piston top opens them, right?
EPH, I have no clue to how much squish this saw should have. I used the squish of the MS460 for a comparison. All this was done in my regular shop where I had a vernier handy to take measurements.
So I used a steel scale instead and found the squish is nearly double that of a MS460.
I often use the 0.8mm gaskets instead of the 0.3mm standard ones to reduce piston damage on old 460s, but at the cost of compression, which often leads to unburned 2 stroke oil.
For the sheer lack of tools at that moment I used up  a couple of depth gauge files and emery paper to lower that cylinder.Well the result was not something to put up in a beauty contest, so I used some Three bond as the leveler and allowed it to dry for about 20 minutes before installing the cylinder.
It was a great relief that there were no leaks.
The first hint of things turning for the better was that the oily carbon blackened spark plug came out looking dry and dark gray after about 5 minutes of test cuts. And, unlike earlier, not a hint of exhaust smoke at anytime.
I tried it with different mixes of two stroke oil, 2% of Stihl oil, 3% of the the local oil and finally 4% of the local oil, the last being the standard out here.
Even with the 4% oil the results are very good.
From a slightly tinny exhaust note the sound has become more throatier.
It has also made the revving more 'linear' (if that is the proper word)
Joe
 

you say you got a vernier , just get some soft solder and place solder in the cylinder thou the spark plug hole , place the solder along the same direction the crankshaft is laying in the saw and turn more over to get a squish reading , If plug is grey I would richen it up abit , more a brown colour is better